The manuscript by Dai et al. reports the observation data by the single particle aerosol mass spectrometer (SPA-MS) at Yangzhou, China. The observation was conducted at around the COVID-19 lockdown (LD) period in 2021. The major finding includes enhanced abundance of aged black carbon (BC) particles during the LD period. The technique for the study sounds. The manuscript is well organized and written as a measurement report. The topic is within the scope of the journal. I suggest the publication of this manuscript after addressing the following comments.

Comments

 Figure 3

Do the authors have BC and CO concentration data? Both these chemical species are emitted from incomplete combustion. Their emission ratios depend on types of sources. If the authors could provide these data, it may help supporting the conclusion that the major emission source of BC during the LD period was different.

Figure 5

The label for x-axis is missing. During the LD period, most of carbon cluster ions were smaller than C₇⁺, while C₈⁺~C₁₁⁺ ions were abundant during the before/after the LD period. Does the change in the carbon cluster ion sizes tell anything about emission sources?

L308

Can these organic ion fragments be produced from POA, SOA, or both of them? Do the authors have any comments?

L492

Could condensation also contribute to the process? Or, do the authors have a strong support to demonstrate that the process was dominantly occurring by aqueous or heterogeneous reactions?

L504-505

Was Yangzhou the only one city which experienced the lockdown in the YRD region during the 3.5 months of the observation period? Could the authors be able to provide some supporting information for the statement?

Dai et al. showed the measurement result of the Black carbon-containing (BCc) particle variation and its sources in Yangzhou, China during the COVID-19 pandemic in 2021 summer. The analysis is very detailed, and the manuscript is not hard to follow. I recommend publishing it as a measurement report on ACP after dealing with the following comments.

#1. Line 29. What kinds of “local gas pollutants”?

#2. Line 28-36. These sentences are organized in a little messy. Need to be rewritten more clearly.

#3. Line 46-50.  These highlights do not match the title and the major objects of this paper. The author should provide some guidance for BCc, rather than a general PM_2.5.

#4. Line 75-76, references for “Due to the complex emissions and feedback with the East Asian monsoon”

#5. Line 122. Any previous results about BCc, as BCc is the topic of your study?

#6. Line 125. You do not conduct satellite measurements. Change to “combine”

#7. Line 148. What is the flow rate and cut size for your cyclone? More detailed information is needed.

#8. Line 168. Figure 1. Remove (b) and the right corner Chinese map is not very clear. Also the author should add some cities mentioned in the text.

#9. Line 178-182. It is very unclear here. Do you mean that you used the MERRA-2 data to replace the dataset from background SO2 from TROPOMI? Also, please provide the link for your data source and change SO2 mass concentration to SO2 column concentration, as satellite only provides column concentration.

#10. Line 242. Remove “Further”

#11. Line 254. Please provide the meaning about the marked region.

#12. Line 259. The resolution of Figure 3 is too low.

#13. Line 262. Which satellite produce of PM2.5 was used? The related information should be provided in section “2.2.1 Satellite Product”

#14. Line 430. Figure 7. It is hard to see the Log-normal distribution.

#15. Line 525. The resolution of Figure 10 is too low.

#16.  Line 541. Figure 11 should be removed and be used as a TOC/Abstract graphic.

#17. Line 567. The resolution of Figure 12 is too low.

#18. Line 643-651. Similar to #3, the author should highlight some discussion about BCc, rather than a general PM_2.5.

Review of "Measurement report: Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China" by Dai et al.

Manuscript number: egusphere-2023-2454

The manuscript "Characteristics of airborne black carbon-containing particles during the 2021 summer COVID-19 lockdown in Yangzhou, China" mainly investigates the changes in black carbon-containing (BCc) particles during the COVID-19 pandemic in the summer of 2021. The authors found that the number fraction of freshly emitted BCc particles decreased to 28% during the lockdown (LD). Meanwhile, the authors observed an increase of more aged BCc particles. Authors conducted ground measurements, spaceborne observations, and mass spectrometric analysis during the COVID-19 2021 summer lockdown in Yangzhou, to investigated the impact of small-scale and short-term stringent emission controls on local ambient aerosol and the mixing state of BCc particles. The authors aim to provide valuable insights for future air pollution control measures. However, COVID-19 lockdown is a special policy made by the government during the special period; meanwhile, Yangzhou is too small to explain well the impact of small-scale and short-term stringent emission controls on local ambient aerosol and the mixing state of BCc particles. Furthermore, there are many ambiguities in the manuscript. Therefore, unfortunately, I do not recommend the manuscript to be published. Please see my comments below.

Major Comments:

1. The authors claimed that “the number fraction of freshly emitted BCc particles decreased to 28% during the lockdown (LD), with that from vehicle emissions experiencing the most substantial reduction”, and also expressed that “the uncontrolled reductions of nitrogen oxides (NOx) and volatile organic compounds (VOCs) likely contributed to increased ozone (O3) concentrations, increased the oxidizing capacity”. My question is what are the emission sources of NOx and VOCs under the scenario of “the vehicle emissions experiencing the most substantial reduction”? Therefore, I think the authors analysis is paradoxical. Therefore, the mechanism of increase of more aged BCs particles is unreasonable.
2. During the observation period, precipitation occurred intermittently (Figure 2b), and the author only mentioned “the data collected during the precipitation were excluded from the analysis” in Section 3.1. This method is obviously not enough to eliminate the impact of precipitation. Additionally, the maximum daily precipitation at the observation site does not exceed 10 mm (Figure 2b), however, the author has repeatedly mentioned heavy precipitation. How is the degradation of precipitation defined?
3. In Section 3.5, the authors propose a method to roughly estimate the local and non-local proportions for each type of BCc particles in Yangzhou only during the ALD period. And the distribution of BCc particles during LD is unclear and there is no comparison before and after. The manuscript focuses on the COVID-19 lockdown, how do the authors provide valuable insights for future air pollution control measures? As the authors pointed out, “Since there was a heavy precipitation on July 28th (the day before lockdown) which removed most atmospheric pollutants, the air pollutants might be influenced mostly by regional transport as local emissions were significantly cut down during the LD period”. Therefore, is it necessary to analyze the distribution of BCc particles during LD period?

Minor Comments:

1. A detailed description of the shading in Figure 1 should be given.
2. Line 177-181, NO2 (NRTI/L3 NO2) obtained from the TROPOspheric Monitoring Instrument (TROPOMI) with a spatial resolution of 3.5×7 km²; Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2 SO2SMASS) with a spatial resolution of 69×55 km². My question is, how do the authors think about the impacts of resolution difference between two datasets on the results?
3. Line 228, “dramatically”, the authors should objectively describe and carefully consider the modifiers.
4. Line 244-246, “surface O3 concentration showed an increase of 12.6 μg m-³ (19%) during the LD period compare to the BLD period, which may attribute to the reduction of fresh NO emissions that alleviates O3 titration”. In fact, the O3 concentration showed sustained higher values during the ALD period compare with those in BLD and LD periods (Figure 3). What is the reason?
5. Line 246-248, “However, the average concentrations of PM2.5 (19.9 vs. 21.2 μg m-³) and SO2 (9.4 vs. 9.5 μg m-³) were very close between BLD and LD stages (Figure 3).” According to this logic, TVOC was also very close between BLD and LD stages (Figure 3). Why was it not mentioned?
6. Line 254, There is a discrepancy between the segmentation of the chart and the textual expression. Additionally, are all observation elements conducted at a rooftop laboratory 20 m above ground? If yes, is it appropriate to use “surface”?
7. Line 270-273, “Such results highlight the short-term, limited-scale, and human-induced reduction in air pollutants as a result of the lockdown measures in Yangzhou, and demonstrate the effectiveness of regional stringent emission control in reducing local atmospheric pollutant concentrations”. From the analysis of the results, it is obvious that the result cannot be obtained. Furthermore, this conclusion is completely opposite to that in Section “Abstract” (Line 46-50).
8. Line275-280, Calculations were only conducted for the regions with PM2.5 > 10 µg m^-3, NO2 > 0.2 Dobson units (DU), and SO2 > 0.2 DU in the BLD period. What is the reason for defining these thresholds? Please provide an explanation.
9. Line 297, Figure 5, please add a horizontal axis identifier.
10. Line 354, As shown in Figure 1, it can be seen that significant precipitation occurs during the LD period. The authors’ expression here is “the day before lockdown”, please unify it.
11. Line 462, 477, RH is a key element which is responsible for the formation of BCOC and BC-SNA particles. Is there a simple positive or negative correlation between them?
12. Line 474-477, “As shown in Figure 9, BCc particles with ~400 nm Dva exhibited significant diurnal fluctuations in the OC/Cn and SNA/Cn ratios, during the LD period. Moreover, there was a noticeable increase in the proportion of BC-SNA particles during nighttime when RH was relatively high”. Compared with the LD period, BCc particles exhibited more significant diurnal fluctuations in the OC/Cn and SNA/Cn ratios during the ALD period. What is the reason?
13. Line 490-491, Please confirm the relationship between “BCc particles” and “RH” again to prevent conflicting results.